Interrupt handling

cysignals provides two related mechanisms to deal with interrupts:

  • Use sig_check() if you are writing mixed Cython/Python code. Typically this is code with (nested) loops where every individual statement takes little time.
  • Use sig_on() and sig_off() if you are calling external C libraries or inside pure Cython code (without any Python functions) where even an individual statement, like a library call, can take a long time.

The functions sig_check(), sig_on() and sig_off() can be put in all kinds of Cython functions: def, cdef or cpdef. You cannot put them in pure Python code (files with extension .py).

Basic example

The sig_check() in the loop below ensures that the loop can be interrupted by CTRL-C:

from cysignals.signals cimport sig_check
from libc.math cimport sin

def sine_sum(double x, long count):
    cdef double s = 0
    for i in range(count):
        sig_check()
        s += sin(i*x)
    return s

See the example directory for this complete working example.

Note

Cython cdef or cpdef functions with a return type (like cdef int myfunc():) need to have an except value to propagate exceptions. Remember this whenever you write sig_check() or sig_on() inside such a function, otherwise you will see a message like Exception KeyboardInterrupt: KeyboardInterrupt() in <function name> ignored.

Using sig_check()

sig_check() can be used to check for pending interrupts. If an interrupt happens during the execution of C or Cython code, it will be caught by the next sig_check(), the next sig_on() or possibly the next Python statement. With the latter we mean that certain Python statements also check for interrupts, an example of this is the print statement. The following loop can be interrupted:

>>> while True:
...     print("Hello")

The typical use case for sig_check() is within tight loops doing complicated stuff (mixed Python and Cython code, potentially raising exceptions). It is reasonably safe to use and gives a lot of control, because in your Cython code, a KeyboardInterrupt can only be raised during sig_check():

from cysignals.signals cimport sig_check
def sig_check_example():
    for x in foo:
        # (one loop iteration which does not take a long time)
        sig_check()

This KeyboardInterrupt is treated like any other Python exception and can be handled as usual:

from cysignals.signals cimport sig_check
def catch_interrupts():
    try:
        while some_condition():
            sig_check()
            do_something()
    except KeyboardInterrupt:
        # (handle interrupt)

Of course, you can also put the try/except inside the loop in the example above.

The function sig_check() is an extremely fast inline function which should have no measurable effect on performance.

Using sig_on() and sig_off()

Another mechanism for interrupt handling is the pair of functions sig_on() and sig_off(). It is more powerful than sig_check() but also a lot more dangerous. You should put sig_on() before and sig_off() after any Cython code which could potentially take a long time. These two must always be called in pairs, i.e. every sig_on() must be matched by a closing sig_off().

In practice your function will probably look like:

from cysignals.signals cimport sig_on, sig_off
def sig_example():
    # (some harmless initialization)
    sig_on()
    # (a long computation here, potentially calling a C library)
    sig_off()
    # (some harmless post-processing)
    return something

It is possible to put sig_on() and sig_off() in different functions, provided that sig_off() is called before the function which calls sig_on() returns. The following code is invalid:

# INVALID code because we return from function foo()
# without calling sig_off() first.
cdef foo():
    sig_on()

def f1():
    foo()
    sig_off()

But the following is valid since you cannot call foo interactively:

from cysignals.signals cimport sig_on, sig_off

cdef int foo():
    sig_off()
    return 2+2

def f1():
    sig_on()
    return foo()

For clarity however, it is best to avoid this.

A common mistake is to put sig_off() towards the end of a function (before the return) when the function has multiple return statements. So make sure there is a sig_off() before every return (and also before every raise).

Warning

The code inside sig_on() should be pure C or Cython code. If you call any Python code or manipulate any Python object (even something trivial like x = []), an interrupt can mess up Python’s internal state. When in doubt, try to use sig_check() instead.

Also, when an interrupt occurs inside sig_on(), code execution immediately stops without cleaning up. For example, any memory allocated inside sig_on() is lost. See Signal handling without exceptions for ways to deal with this.

When the user presses CTRL-C inside sig_on(), execution will jump back to sig_on() (the first one if there is a stack) and sig_on() will raise KeyboardInterrupt. As with sig_check(), this exception can be handled in the usual way:

from cysignals.signals cimport sig_on, sig_off
def catch_interrupts():
    try:
        sig_on()  # This must be INSIDE the try
        # (some long computation)
        sig_off()
    except KeyboardInterrupt:
        # (handle interrupt)

It is possible to stack sig_on() and sig_off(). If you do this, the effect is exactly the same as if only the outer sig_on()/sig_off() was there. The inner ones will just change a reference counter and otherwise do nothing. Make sure that the number of sig_on() calls equal the number of sig_off() calls:

from cysignals.signals cimport sig_on, sig_off

def f1():
    sig_on()
    x = f2()
    sig_off()

cdef f2():
    sig_on()
    # ...
    sig_off()
    return ans

Extra care must be taken with exceptions raised inside sig_on(). The problem is that, if you do not do anything special, the sig_off() will never be called if there is an exception. If you need to raise an exception yourself, call a sig_off() before it:

from cysignals.signals cimport sig_on, sig_off
def raising_an_exception():
    sig_on()
    # (some long computation)
    if (something_failed):
        sig_off()
        raise RuntimeError("something failed")
    # (some more computation)
    sig_off()
    return something

Alternatively, you can use try/finally which will also catch exceptions raised by subroutines inside the try:

from cysignals.signals cimport sig_on, sig_off
def try_finally_example():
    sig_on()  # This must be OUTSIDE the try
    try:
        # (some long computation, potentially raising exceptions)
        return something
    finally:
        sig_off()

If you also want to catch this exception, you need a nested try:

from cysignals.signals cimport sig_on, sig_off
def try_finally_and_catch_example():
    try:
        sig_on()
        try:
            # (some long computation, potentially raising exceptions)
        finally:
            sig_off()
    except Exception:
        print("Trouble! Trouble!")

sig_on() is implemented using the C library call setjmp() which takes a very small but still measurable amount of time. In very time-critical code, one can conditionally call sig_on() and sig_off():

from cysignals.signals cimport sig_on, sig_off
def conditional_sig_on_example(long n):
    if n > 100:
        sig_on()
    # (do something depending on n)
    if n > 100:
        sig_off()

This should only be needed if both the check (n > 100 in the example) and the code inside the sig_on() block take very little time.